This invention relates to a method of forming superconducting tape of a high temperature oxide superconductor, and in particular to a tape having a silver cladding or sheath.
A new superconductor comprised of oxides of lanthanum, barium, and copper, and having a critical temperature of 30 Kelvin, K, was discovered at the Zurich laboratory of International Business Machines Corporation, J. G. Bednorz et al., "Possible High-T.sub.c Superconductivity in the Ba-La-Cu-O System," Z. Phys. V. 64, 189 1987. The discovery of the lanthanum-barium-copper-oxide system led to the discovery of a series of superconducting systems having a high critical temperature above 30 K. Exemplary of the newly discovered systems are, lanthanum-strontium-copper-oxide , yttrium-barium-copper-oxide, bismuth-strontium-calcium-copper-oxide, and thalium-barium-calcium-copper-oxide. The superconductors in the above systems, having a critical temperature above 30 K, are herein referred to as high temperature oxide superconductors or oxide superconductors. Additional information about the oxide superconductors and methods of forming powders or films thereof can be found, for example, in the following disclosures incorporated herein by reference, "Chemical Engineering and the Development of Hot Superconductors," R. Kumar, Chemical Engineering Progress, pp.17-27, April 1990, and "Chemistry of High-Temperature Superconductors," A. W. Sleight, Science, Vol. 242, pp.1519-1527, Dec. 1988.
The discovery of the oxide superconductors enables the development of superconducting equipment operated under liquid nitrogen cooling, about 77 K, instead of liquid helium cooling, about 4.2 K. Among the applications for such oxide superconductors are power transmission lines, rotating machinery, superconducting magnets, e.g. for, fusion generators, particle accelerators, levitated vehicles, magnetic separation, energy storage, and magnetic resonance imaging. These devices require the development of wire or tape of the oxide superconductors.
Owing to the brittleness and reactivity of oxide superconductors, it is advantageous to form metal-ceramic composites of metal-clad oxide superconductor wires, tapes, and multifilaments, so that the metal claddings malleability compensates for the ceramics brittleness. Furthermore, the metal provides a good means of thermal dissipation, stabilizing the superconductor environment. The stabilizing effect is of fundamental importance for Type II superconductors, in which undesirably large local rises in temperature can develop through flux jumping in the mixed state. Flux jumping is a sudden, localized flux motion resulting in a local rise in temperature. Unfortunately, the Type II oxide superconductors, in particular, bismuth-lead-strontium-calcium-copper-oxide, react with nearly every metal with which they come into contact.
Silver has been found to be compatible with and non-poisoning to the superconducting systems yttrium-barium-copper-oxide, thallium-barium-calcium-copper-oxide, and bismuth-strontium-calcium-copper-oxide. It is also known that lead can be added to the bismuth system in order to stabilize the phase having a critical temperature of 110 K, comprised of bismuth, strontium, calcium, and copper oxides in the ratio of about 2:2:2:3, respectively. Such superconductor compositions and phases are sometimes herein shown by a formula, for example, Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10+y. The cation ratios given in such formulas usually indicate the ideal structure, and the exact oxygen content per molecular unit is nominal so that y varies within about a fraction of one. In addition, there may be substitution of cations on other sites, cation vacancies, or oxygen interstitials present so that the actual superconducting phase is not exactly that given by the ideal formula.
One of the most promising methods of forming oxide superconductors into long wires or tapes is the powder in tube drawing-rolling process, for example, see "Development of High-T.sub.c Superconducting Wire with High Critical Current Density", T. Matsumoto, K. Aihara, M. Seido, Hitachi Review, Vol. 39, (1990), No. 1, pp. 55-62, incorporated herein by reference. Briefly described, in this method superconducting powder was put into a 300 millimeter long silver tube with a 6 millimeter diameter and 0.5 millimeter wall thickness. The filled tube was drawn in 30 repetitions to a diameter of 2.8 millimeters. The drawn tubing was rolled into 6 millimeter wide tape 0.05 to 0.5 millimeters in thickness. For example, rolling was repeated about 100 times for a tape of 0.01 millimeter thickness. The tape was heat treated at 910.degree. C. for twenty hours to react and sinter the core to form a continuous superconducting core.
It is an object of this invention to provide a method of forming a silver clad tape of the oxide superconductors that does not require extensive drawing and rolling operations.